In recent years, an image display apparatus using a laser as a light source, such as a projector or a liquid crystal television, is under research and development. A laser light source close to an ideal point light source is capable of efficiently condensing light into a small area. Accordingly, it is possible to scale down the optical system of a projector and implement the small-sized projector. In addition, if linearly polarized laser light is used for the back light of a liquid crystal television, the liquid crystal television having high light use efficiency and low power consumption can be implemented. The reason for this is as follows. Since a liquid crystal panel needs linearly polarized light to be incident thereon, when a lamp or an LED which emits randomly polarized light is used as a light source, a conventional liquid crystal television has had a polarized light filter which converts the randomly polarized light to linearly polarized light and, if linearly polarized laser light is used as a light source, it is possible to remove the polarized light filter from the liquid crystal television and suppress a light loss.
When a laser light source is used as the light source of such an image display apparatus, laser light sources for red, green, and blue colors that are the three primary colors of light are needed. However, while high-output red and blue laser light sources have been implemented by semiconductor lasers, a high-output green laser light source is hard to implement since it is difficult to form a practically optimum material that can be used to form a semiconductor laser for the high-output green laser light source. Therefore, attention has been given to, e.g., a wavelength conversion device which wavelength-converts fundamental light from a solid-state laser to a harmonic wave using a wavelength conversion element to output high-output green laser light and the development of the wavelength conversion device directed toward the large-scale production thereof has been promoted. The solid-state laser indicates a structure which uses a laser medium to obtain laser light, and examples thereof include a semiconductor laser excitation solid-state laser which achieves excitation using a semiconductor laser.
FIG. 15 is a plan view showing a schematic structure of a conventional wavelength conversion device 100. The conventional wavelength conversion device 100 shown in FIG. 15 includes an excitation laser light source 110, a condenser lens 110c, a laser medium 120, a concave mirror 200, two resonator mirrors 130 (130a and 130b), and a wavelength conversion element 140. Excitation light 110a emitted from the excitation laser light source 110 is condensed by the condenser lens 110c to be incident on the laser medium 120. The laser medium 120 absorbs the excitation light 110a and generates fundamental light 120a using the two resonator mirrors 130 (130a and 130b). The wavelength conversion element 140 is disposed between the two resonator mirrors 130 (130a and 130b) to wavelength-convert the fundamental light 120a to harmonic light 160. Note that each of the components is disposed on and fixed to a base stand 100a of the wavelength conversion device 100. As shown in FIG. 15, the resonator mirror 130a which is one of the two resonator mirrors 130 (130a and 130b) for resonating the fundamental light 120a uses an end surface 300 formed of the curved surface of the concave mirror 200. The conventional wavelength conversion device 100 has a large number of parts to result in the problem of high cost. Therefore, it has been proposed to form the resonator mirror 130a not on the end surface 300 of the concave mirror 200, but on an end surface of the wavelength conversion element 140 and remove the concave mirror 200.
However, in the case where the resonator mirror 130a is formed on the end surface of the wavelength conversion element 140, the problem arises that the efficiency of conversion (hereinafter referred to as electricity-to-light conversion efficiency) from power input to the excitation laser light source to the harmonic light 160 decreases compared with that in the conventional wavelength conversion device 100.
To implement a green laser light source having high electricity-to-light conversion efficiency and low power consumption, it is needed to efficiently convert the fundamental light to the harmonic light.
To satisfy the need, there is a method which heats the input terminal portion of a laser medium that outputs fundamental light to cause a change in the refractivity of the laser medium and converges the fundamental light propagating a wavelength conversion element by use of the refractivity change. It is shown that, thus, in the wavelength conversion element, an optical output per unit cross-sectional area in a plane perpendicular to the optical axis of the propagating fundamental light increases to increase a non-linear effect and a high efficiency of conversion to the harmonic light can be obtained (see, e.g., Patent Document 1).
In an image display apparatus, when the high-efficiency green laser light source thus obtained is used, it is important in maintaining high-quality display of a displayed image to operate the high-efficiency green laser light source while stabilizing an output of green laser light at a given value. Therefore, a high-brightness and high-definition image display apparatus according to a field sequential method is proposed in which a plurality of green laser light sources each using a wavelength conversion element are electrically controlled using a drive control device (see, e.g., Patent Document 2).
However, in the technology described above, the rising edge of the harmonic light thus obtained is not steep. As a result, if the harmonic light is to be used for the image display apparatus without any modification, the problem is encountered that it is difficult to obtain a high-brightness image display apparatus. In addition, since the rising edge is not steep, the problem is also encountered that gradation control is difficult and it is hard to obtain a high definition image.    Patent Document 1: Japanese Patent Application Laid-open No. H2-146784    Patent Document 2: Japanese Patent Application Laid-open No. 2008-250037